Communication systems are often subject to repetitive background noise. For example, automobile muffler systems, machines on a production floor, engines in a vehicle, boat and airplane, or any other source of repetitive noise can interfere with an acoustic or other pick-up, such as a microphone, hand set of a telephone, hydrophone, vibration sensor, or electronic transducer located near the noise source. In particular, microphones in emergency vehicle communication systems associated with police cars, fire trucks and ambulances pick-up not only a user's voice, but also a repetitive background noise generated by the emergency vehicle siren. This repetitive background noise can often overpower a user's voice so that a user's message is difficult to understand. If voice-activated communication systems are employed, background noise increases the difficulty in recognizing voice commands for automatically turning on and off the communication system.
Microphone noise rejection systems have been developed to minimize the level of background noise relative to the level of the desired information or voice signal. Such noise rejection systems typically comprise dual microphones in which a first microphone primarily receives background noise and a second microphone primarily receives both background noise and an information or voice signal. The noise signal is then added to or subtracted from the information signal in order to cancel noise from the information signal. For example, U.S. Pat. No. 5,381,473 issued to Andrea shows a noise-rejection system that uses two microphones to generate respectively a noise signal and a source signal. The noise and source signals are supplied to a differential amplifier to cancel noise from the source signal. The phases of the noise and source signals must be tightly controlled relative to each other in order to successfully remove unwanted noise from the source signal. Unfortunately, this tight phase control is difficult to achieve because the phases of the noise and source signals are extremely sensitive to slight variation in the length of each signal path from the signal source to the noise rejection processing circuitry. The present invention overcomes the phase control problem by an apparatus and method which is independent of the path length of the signals from the signal source to the noise rejection circuitry.
It is therefore an object of the present invention to substantially eliminate repetitive background noise from an information or voice signal without the disadvantages inherent in the prior approaches to noise rejection.